Simian virus 40 (SV40) transforms certain cells into cancerous cells, and one to ten copies of SV40 genomes are incorporated into host cell DNA. Only A gene, the gene expressed during the "early" phase of productive infection, is expressed in transformed cells. Such transcriptional specificity can be demonstrated with isolated transformed cell chromatin. The regulation of SV40 gene expression in chromatin is being investigated by chromatin reconstitution method. For example, chromatin can be dissociated and reassembled in the presence of NaCl and urea, and reconstituted chromatin shows transcriptional specificity for SV40 genes very similar to native chromatin. Using this method we are investigating the possible presence of SV40 specific proteins in transformed cell chromatin but not in normal cell chromatin, and the mode of regulation of SV40 genes by chromosomal proteins.